The thesis will discuss how small objects such as bacteria and viruses can be manipulated on-chip using light via Photonic crystals (PhCs). In particular, PhCs could be used in the context of novel fast antimicrobial susceptibility tests (ASTs). We will see how point-like defects in the lattice structures form resonant cavities. The supported optical mode of these cavities is extremely spatially confined, thus generating large field gradients that allow the trapping of small polarisable objects (10 - 5000 nm) in their proximity. In contrast to classical optical tweezers, photonic crystal cavities do not require a bulk lens system for focusing and are, therefore, not limited by diffraction. Furthermore, when an object is trapped, the resonance is modified according to the refractive index and size of the object. Thus, they are true sensors capable of autonomously trapping their own analytes. The PhCs will be integrated into a microfluidic device to direct suspended particles to the cavity location for trapping. The so-formed optofluidic chip is mounted on an end-fire transmission setup to excite the cavity mode and read out its perturbation. The thesis defines the first steps towards more versatile and faster AST using photonic crystal cavities. Indeed, the rising threat of AMR is pushing the community to develop new techniques to overcome the long time to results challenge of currently accepted AST techniques. In this work, we exploit resonant 2D photonic crystal cavities to trap and sense single bacteria when healthy or exposed to antibiotics or bacteriophages. The technique allows the real-time monitoring of bacterium-antimicrobial interactions independently from bacterial growth.